1. Technical Field of the Invention
The present invention relates generally to an improved structure of a ceramic heater designed to ensure the reliability of an electrical joint of an external connector to a body of the ceramic heater and a gas sensor quipped with such a ceramic heater.
2. Background Art
There are known gas sensors which are installed in an exhaust pipe of automotive engines to determine an air-fuel ratio of mixture for combustion control in the engine to enhance the efficiency of purifying exhaust emissions through a three-way catalytic converter installed in the exhaust pipe. Gas sensors of this type typically include a sensor element made of a solid electrolyte body possessing an oxygen ion conductivity. The sensor element usually has installed therein a ceramic heater which works to heat a body of the sensor element up to an activation temperature in order to measure the concentration of a gas correctly.
Japanese Patent First Publication No. 11-292649 (U.S. Pat. No. 6,121,590 and U.S. Pat. No. 6,118,110) assigned to the same assignee as that of this application discloses a typical ceramic heater for use in sensor elements of the type, as described above. FIG. 14 is a partially sectional view showing the ceramic heater.
The ceramic heater includes a ceramic body 91 and ceramic-metal connector assemblies 9 (only one is shown for the brevity of illustration). Each of the connector assemblies 9 consists of a metallic layer 92 affixed to the surface of the ceramic body 91, a connector terminal 94, and a joint layer 93 formed on the metallic layer 92 to make an electrical connection with the connector terminal 94. When the connector assemblies 9 are exposed to the gas for a long period of time, it may result in oxidization of the connector terminals 94, which causes the joint layers 93 to peel off the metallic layers 92, thus leading to disconnection of the connector terminals 94 from the ceramic body 91. In order to avoid this problem, the connector terminals 94 and the joint layers 93 are coated with electroless plated layers 95, respectively, which are each made of nickel or nickel-boron.
In recent years, the temperature of exhaust gas of automotive engines has been increased in order to meet legal requirements of emission control. This may result in erosion of the electroless plated layers 95 and oxidization of the connector terminals 94, thereby leading to disconnection of the connector terminals 94 from the ceramic body 91.
When a gas sensor equipped with the above type of ceramic is installed in an exhaust pipe of automotive engines, NOx contained in exhaust gasses may leak inside the gas sensor and react with moisture, which will be produced in a cold condition of the engine when at rest, to produce nitric acid. The nitric acid usually will be a cause of erosion of the Ni-plated layers 95, thus resulting in disconnection of the connector terminals 94 from the ceramic body 91.